1. Field of the Invention
This invention has to do with seismic surveying using shear waves. More specifically it is directed to methods and apparatuses for generating shear waves wherein substantially no pressure wave is generated in a selected plane.
2. Description of the Prior Art
In a shear wave, the motion of the earth particles is normal to the direction of propagation of the wve. The advantages of using shear waves for seismic prospecting are well known in the art. At a given frequency shear waves travel through the earth at a speed only a fraction of the speed of pressure waves so that the shear waves have shorter wave lengths than those of pressure waves having the same frequency. This results in better resolution in seismic profiles.
Many shear wave seismic sources have been proposed including sources that may be used in shot holes. In U.S. Pat. No. 2,880,816 (1959), Widess et al disclosed that ordinarily the detonation of a charge in the earth creates little downward traveling seismic shear wave energy because the horizontal movement of earth particles caused by the detonation is symmetrical about a vertical axis or plane through the charge. For the detonation to generate significant downward traveling shear wave energy, this movement should be asymmetrical. Accordingly, Widess et al proposed detonating asymmetrically a symmetric explosive charge such as an elongated explosive column ignited at one end of the column, or a block of explosive with a concave face ignited at a point on the surface of the block on the other side of the concave face. Alternatively, a symmetric explosive charge may be detonated at a boundary between two media of substantially different wave transmitting properties. All the shear wave sources proposed by Widess, however, appear to generate pressure waves in all directions and in all planes reached by the seismic waves.
Trulio et al, in U.S. Pat. No. 4,103,756 (1978), proposed a rotational device which may be used in bore holes to generate pressure and shear waves. The device comprises a striker having a number of mass elements collectively rotatable about the bore hole axis. At selected rotational speeds, the mass elements are released so that they angularly impact the periphery of the bore hole at points circularly distributed along the periphery. The mass elements are equipped with teeth to grip the bore hole periphery upon impact thereby generating both pressure and shear waves. The pressure waves generated by the device appear to propagate in all directions and in all planes reached by the seismic waves.
An air gun device is proposed by Murphy in U.S. Pat. No. 4,038,631 (1977) for use in bore holes for generating pressure waves and shear waves. The air gun device comprises an elongated cylinder having a single discharge port opening out radially on one side only for abruptly releasing compressed air in a direction perpendicular to the axis of the air gun. Firing of the air gun generates pressure waves and shear waves in a zone of earth surrounding the bore hole. As pressurized air is discharged from the vent port of the air gun to exert a force on the bore hole wall for generating seismic waves the air gun recoils according to Newton's Third Law of Motion. Different parts of the air gun may be caused to hit the borehole wall at different locations, generating shear waves in a random fashion. Since the impact of the air gun on the borehole wall caused by recoil is comparable in strength to the impact of the pressurized air, the shear waves generated by recoil have relatively significant amplitudes and will appear as high noise levels in the shear wave record. Murphy et al have not disclosed how the recoil of the air gun may be controlled or reduced to alleviate this problem.
Kitsunezaki, in U.S. Pat. No. 4,207,961 (1980), disclosed a shear wave logging method. A magnetic field is formed by a permanent magnet and a iron yoke suspended in a borehole filled with water. A bobbin assembly equipped with coils is disposed in the magnetic field so that it may slide with respect to the magnet and the yoke along a line perpendicular to the axis of the borehole. If current is passed through the coils, the bobbin assembly will move in either one of two opposite directions of the line, depending on the direction of current. During its motion, the bobbin assembly ejects a volume of water on one side and sucks in the same volume of water on the opposite side. It is stated that since there is no net change in volume of the water, no pressure wave is generated. But the ejection and suction of water create positive and negative pressures on the borehole wall, which will generate waves. Recoil causes the permanent magnet and the yoke to move in an opposite direction to that of the bobbin assembly. A heavy mass is attached to the magnet and the yoke to reduce their recoil displacements. Such a recoil reducing method may, however, be impractical if strong recoil forces are generated, as in shear wave seismic prospecting.